Hi and welcome to the Chiari forum.
I am not sure if I understand your question...are you asking about the bone removed and is not replaced and the risk should you fall post op?
If that is what you are asking then ask your Dr that.....I will say, I fell before surgery with regularity....I have not fallen since surgery.....my balance has been so much improved as well as other issues I had pre op.....
Make sure ALL related conditions are ruled out b4 surgery and ask why you are a surgical candidate....if they say it is the size of herniation....then ask if you have a CSF obstruction...if he /she isn't sure, then you may want to look for a true Chiari specialist.....